II.3. BRASSICA CROPS (BRASSICA SPP.) – 189
in cabbage, cauliflower, kale and kohlrabi. The ratio of male to female in hybrid
production fields is normally 1:1 or 1:2 (Takahashi, 1987).
Most cabbage and cole crops and some Asian vegetables are biennials and will not
bolt until they have been exposed to temperatures of 4-7°C for 6-8 weeks. Day length has
no affect on bolting or flower initiation (Nieuwhof, 1969). At the end of the first year,
cabbage plants can withstand temperatures of -12°C to -14°C for extended periods, but
lower temperatures can cause much damage, as can alternating periods of frost and thaw
(Nieuwhof, 1969). The usual practice in producing cabbage seed is to sow in the summer
with the plants over-wintering, bolting in the spring and to harvest the seed in summer.
Cultivars differ in their winter hardiness with red cabbages the least hardy and savoy the
hardiest. Summer temperatures are also important in determining seed yield.
Temperatures above 25°C arrest growth and cause seed abortion. Because of these
environmental constraints, commercial production tends to be concentrated in areas with
mild winters, sufficiently cold to ensure vernalization without winter kill, combined with
moderate summer temperatures. The availability of irrigation is also important to
obtaining uniform high yields.
Seeding of the biennial crops in the northern hemisphere is normally done in
mid-June to mid-August. If the seed is to be sown in beds for transplanting, rather than
direct seeding into the field, seeding should be done about ten days earlier than the field
sowing to allow for the plant setback brought on by transplanting (Nieuwhof, 1969).
The recommended rate for field sowing is 3-5 kg per hectare, unless precision sowing is
practiced, where only 1-2 kg per hectare is needed. Plants are thinned to 35-40 cm
between plants within the row. To increase the over-wintering survival rate, plants may
be earthed up covering the most sensitive plant portion just below the head. Weed control
is critical, as in mild winters weeds may over grow the crop. Most of the cole crops are
self-incompatible and depend on insects, primarily honey bees, to effect fertilisation.
Harvesting is done once the pods have turned yellow and the seedsbrown. Depending on
field size and seed value, harvesting may be done by various methods from hand cutting
and threshing to straight combining. Kohlrabi, although a true biennial, can be vernalized
by initiating germination through pre-soaking the seed for 8-9 hours at 20ºC followed by
a cold treatment of -1ºC for 35-50 days. The treated seed can then be sown directly into
the field in the spring and the seed crop harvested in the fall. Brussels sprouts and kale are
grown for seed in the same manner as cabbages.
For cauliflower and broccoli crops, only a mild vernalization period is required so
environmental limitations are less stringent. However, as a seed crop, these forms
normally require an extended growing season. Selection of cauliflower varieties for a
tighter curd has resulted in slow and incomplete bolting, thus further extending the
required growing season. In Western Europe, cauliflower is sown in September and
over-wintered under glass with transplanting to the field in early spring. Transplants are
spaced on a 50 × 50 cm or smaller grid. Flowering occurs in July or August and the crop
is harvested in September or early October. Seed production of tropical and subtropical
cauliflower is discussed by Lal (1993).
Drying the harvested Brassica vegetable seed is frequently required. To maintain
germination capacity, the maximum air drying temperature should not exceed 60ºC.
If seed is to be stored for a year, maximum moisture content should not exceed 9% with a
storage temperature of 5-10ºC.
The biennial turnips and Swedes (rutabaga) regenerate from growing points at or near
ground level. This means they can benefit from a large underground source of nutrients